1. Field of the Invention
This invention relates to a resin for use in manufacturing wet friction materials (hereinafter, referred to as a “resin for wet friction materials”), a manufacturing method thereof, and a wet friction material.
2. Description of the Prior Art
In automatic transmissions of vehicles and the like, wet friction materials are used, for example, in a wet frictional engagement device having a clutch which is operative in oil. A typical example of such a wet friction material is a paper-based wet friction material (which is also called as a paper friction material). Such a wet friction material is formed into a porous material so as to be able to contain a large amount of automatic transmission fluid (ATF). To enhance durability of such a wet friction material, it is required to have uniformly distributed micro pores due to the following reasons. Namely, in a wet frictional engagement device using such a wet friction material, at a time of sliding engagement the wet friction material is compressed by several μm to several tens μm to thereby discharge ATF retained therein through the pores, and is decompressed by absorbing ATF through the pores at a time of release. When ATF is discharged from the inside of the wet friction material, heat generated at its surface due to sliding is dissipated, thereby lowering a surface temperature which has been elevated during sliding. In this way, heat generated due to sliding is dissipated to prevent heat from accumulating in the wet friction material, thereby enabling to provide a cooling effect. This cooling effect significantly affects durability of the wet friction material. Therefore, in order to enhance durability of a wet friction material, it is necessary for the wet friction material to have uniformly distributed pores for enabling to discharge and absorb ATF appropriately.
A conventional wet friction material is composed of a paper body made of a fibrous base material and a resin for wet friction materials. The wet friction material is manufactured by impregnating the paper body with the resin dissolved in an organic solvent, and then drying and thermosetting the resin impregnated paper body. Here, the resin acts as a binder for fibers of the paper body.
However, the conventional wet friction material which is manufactured using the resin dissolved with the organic solvent necessarily accompanies migration of the binder (that is, the resin). As a result, there is a tendency that a concentration of the binder becomes high in a vicinity of the surface area of the paper body while becomes low in the central area of the paper body in a thickness direction thereof. Such non-uniform distribution of the binder results in relatively smaller number of pores in the surface area of the wet friction material. Such wet friction material having relatively smaller number of pores in the surface area thereof is quite disadvantageous for increasing the cooling efficiency (cooling effect) of the wet friction material, since the cooling efficiency can be enhanced by absorbing and discharging ATF through large number of pores as described above. In view of this problem, it is desired to provide a wet friction material having uniformly distributed pores by making the concentration of the binder uniform in a thickness direction of the material. Thus, a strong demand exists for a binder that can contribute to achieve the desire, that is a binder that can be impregnated into a paper body of a wet friction material with uniform concentration.
In addition, in recent years, great attention has been paid to energy conservation and reduction in environmental impacts throughout world. Under the circumstances, in the field of these wet friction materials, a demand for water-soluble phenolic resins for wet friction materials which do not use any organic solvent has been increased from the following reason.
Namely, a conventional wet friction material is manufactured using a resin dissolved with an organic solvent. In the manufacturing process, that resin is used by being diluted with the organic solvent to reach a target concentration. The used organic solvent is then volatilized in drying and thermosetting steps, and this volatilized organic solvent is collected to be disposed of by incineration. However, the incineration of the organic solvent not only consumes energy but also is harmful to environment. Therefore, in view of promotion of the energy conservation and reduction in environmental impacts, it is one of important issues in this technical field to develop a resin for wet friction materials that does not use such organic solvent, that is a water-soluble phenolic resin for wet friction materials.
As stated above, in this technical field, there is a strong demand for a resin that can simultaneously accomplish both the enhanced durability of wet friction materials and the use of a water-soluble resin for wet friction materials that does not use any organic solvent, but such a resin for wet friction materials has not yet been practically realized.